In general, infrared sensors having high sensitivity are divided into a semiconductor type and a physical-property type, according to functional materials used.
Semiconductor infrared sensors formed from semiconductor materials are the most sensitive, because electron-hole pairs generated by means of energy of infrared light are extracted as an electric signal. However, since electron-hole pairs are generated by low energy, a semiconductor material having a narrow energy gap must be employed. However, when a semiconductor material having a narrow energy gap is employed, electron-hole pairs are generated even at room temperature. Therefore, such a semiconductor material produces large dark current and cannot have high sensitivity, unless it is used while being maintained at the helium temperature or the nitrogen temperature.
Further, at present, semiconductor infrared sensors are used only for astronomic observation, because they are very expensive. Further, since semiconductor infrared sensors have sensitivity only within a certain wavelength range, they have a drawback in that an infrared wavelength cannot be selected freely for high-sensitivity sensing.
A physical-property-type sensor which senses changes in a physical property of a material stemming from temperature variation thereof cannot exhibit high sensitivity unless the sensor is in the form of a thin film. However, such a physical-property-type sensor has a practical sensitivity throughout a wide wavelength range, and its most attractive feature is that the sensor can be used at room temperature, and thus, the sensor can be rendered inexpensive. Typical examples of such a physical-property-type sensor include thermistors, thermo-couples, and pyroelectric sensors.
FIG. 1 includes a graph and a diagram which show the principle of a conventional pyroelectric infrared sensor. In FIG. 1, reference numeral 101 denotes a ferroelectric element; 102 denotes a junction-type FET (JFET); 103 denotes an infrared ray; and 104 denotes a resistor Rs.
As shown in FIG. 1, the pyroelectric infrared sensor utilizes a pyroelectric coefficient, which is a temperature gradient of the spontaneous polarization value Ps of the ferroelectric element. Specifically, as shown in FIG. 1(a), the spontaneous polarization value Ps decreases gradually before reaching the Curie temperature Tc, but decreases at a large gradient in the vicinity of the Curie temperature Tc. When the spontaneous polarization value Ps decreases due to a variation in temperature of the ferroelectric element 101, charge become excessive accordingly. The variation in charge due to the temperature variation is called a pyroelectric coefficient. Pyroelectric infrared sensors, which are configured to extract such excessive charge as a signal, have been widely used.
In the case in which the temperature of the ferroelectric element 101 has changed from T1 to T2, the following relationships hold among electric capacitance C of the ferroelectric element 101, generated charge Q, and generated voltage V.Q=ΔPs/ΔT 
                    V        =                ⁢                  Q          /          C                                        =                ⁢                  Δ          ⁢                                          ⁢                      Ps            /                          (                              Δ                ⁢                                                                  ⁢                                  T                  ·                  C                                            )                                                              =                ⁢                  Δ          ⁢                                          ⁢                      Ps            /                          [                                                (                                      T1                    -                    T2                                    )                                ·                C                            ]                                          
A near-infrared (not greater than 2 microns) photoelectron emission surface formed of a compound semiconductor has been put into practical use. Further, there has been employed a highly sensitive semiconductor photoelectric device (photomultiplier tube) which includes such a photoelectron emission surface and a mechanism for amplifying electrons in vacuum. The semiconductor photoelectric device has a sensitivity at least 10 times that of physical-property-type sensors used in the infrared and far-infrared regions.
However, the quantum efficiency of such a photoelectron emission surface is as low as 0.05%; and there does not exist a photoelectron emission device which has sensitivity for infrared light of 2 microns or higher.
Meanwhile, pyroelectric sensors, which are of a physical-property type and can operate at room temperature, have sensitivity over the entire electromagnetic wave region. Therefore, if a low-noise amplification function can be imparted to such a pyroelectric sensor, an infrared sensor which does not depend on wavelength can be produced.
The technical trend arising in pursuit of a supersensitive infrared sensor can be understood from the case of optical sensors. Most optical sensors are wholly solid sensors formed of semiconductor materials; however, some optical sensors are of a vacuum tube type. In such a vacuum tube type, primary electrons emitted from a photoelectron emission device are caused to collide, to thereby generate secondary electrons, and this operation is repeated to thereby obtain a very high amplification gain without affecting the SIN ratio (photomultiplier tube).
However, even in such a photomultiplier tube, a photoelectric material is semiconductor, and the photoelectric material must be chosen in accordance with the desired energy gap, as in the case of infrared sensing materials. Therefore, many restrictions are imposed; e.g., the photomultiplier tube must be cooled in order to obtain high sensitivity.
An infrared photoelectric surface which has high sensitivity at room temperature cannot be formed from conventional semiconductor materials. Meanwhile, incorporating a function of emitting electrons in vacuum as a part of a sensing function is expected to bring forth considerable progress toward attaining higher sensitivity.